Raman spectroscopy has been utilized for a number of years to identify single molecules from various types of samples. Raman spectroscopy, more particularly, has been utilized to identify the vibrational modes of molecules to distinguish between different molecular species. The probability, however, of a Raman interaction occurring between an excitatory beam of light and an individual molecule in a sample is very low, for instance, 103° cm2 for CN. As such, the use of Raman spectroscopy to identify individual molecules has been relatively limited.
One approach to enhancing the Raman spectroscopy effect is to place the molecules near roughened silver surfaces. The surface enhanced Raman spectroscopy (SERS) effect is related to the phenomenon of plasmon resonance, in which metal nanoparticles exhibit an increased optical resonance in response to incident electromagnetic radiation, due to the collective coupling of conduction electrons in the metal. Attempts at implementing SERS have included coating metal nanoparticles or fabricating rough metal films on the surface of the substrate and then applying a sample to the metal-coated surface.